This invention relates to fuel cells, and, more particularly, to the interconnect layer of a monolithic solid oxide fuel cell.
A fuel cell is a device in which a fuel is electrochemically reacted with an oxidant to produce a DC electrical output. A fuel cell includes an anode that defines a flow passage for the fuel, such as hydrogen or a hydrocarbon, and a cathode that defines a flow passage for the oxidant, such as air or oxygen. An electrolyte separates the anode and the cathode. In one type of fuel cell, the monolithic solid oxide fuel cell, the anode, the cathode, and the electrolyte are thin, layered corrugated structures, and the fuel cell further includes plenum conduits for the introduction and removal of the fuel and oxidant.
Such fuel cells are described in more detail in U.S. Pat. Nos. 4,816,036 and 4,913,982, whose disclosures are incorporated by reference. Briefly, in such a fuel cell the fuel flowing through the anode reacts with oxide ions to produce electrons and water, which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit and thence to the cathode. The electrolyte is a nonconductor of electrons, ensuring that they must pass through the external circuit to do useful work, but permits the oxide ions to pass through from the cathode to the anode.
Each individual anode/electrolyte/cathode cell generates a relatively small voltage. To achieve higher voltages that are practically useful, the individual cells are connected together to form a battery. The monolithic solid oxide fuel cell therefore contains an additional layer, an interconnect layer, between the cathode and the anode of adjacent cells.
The interconnect layer must be electrically conducting and must be formable at temperatures comparable with those required to form the other layers. In the technology described in the '036 and '982 patents, the monolithic solid oxide fuel cell is formed by a powder process, which includes sintering of the assembled structure, preferably in an oxidizing atmosphere. The sintering temperature dictated by the sintering requirements of the anode, the cathode, and the electrolyte is typically about 1400 C.-1500 C. The interconnect layer must be sinterable at this same temperature, to a sufficiently good electrical conductivity and a relatively high density of at least about 94 percent of theoretical density, and without chemical interdiffusion with the neighboring layers.
The preferred interconnect material in the past has been magnesium-doped lanthanum chromite. However, this material is not fully satisfactory in some circumstances because it may not sinter to a sufficiently high density at the sintering temperature of the anode, the cathode, and the electrolyte. In order to sinter lanthanum chromite to high densities, firing temperatures greater than 1600 C. and an atmosphere having a low oxygen partial pressure are required. These conditions, however, are not satisfactory for other fuel cell components. At such high temperatures, diffusion and reaction between components becomes significant. Moreover, low oxygen partial pressures cause the decomposition of other materials used in the fuel cell.
There is therefore a need for an improved interconnect approach for use in monolithic solid oxide fuel cells and related types of devices. The present invention fulfills this need, and further provides related advantages.